1. Field of the Invention
The present invention relates to a push-pull converter, in particular for driving cold-cathode fluorescent lamps (CCFLs).
2. Description of the Related Art
As is known, push-pull converters, whether of the current-source type or of the voltage-source type, are widely used in industrial power electronics applications and, in particular, in the lighting field, for driving emergency lamps or cold-cathode fluorescent lamps (CCFLs), the latter being commonly used for back-lighting of the screens of portable computers and for illuminating advertising signs.
The ensuing description will refer, without any loss of generality, to the use of a push-pull converter of current-source type for driving a cold-cathode fluorescent lamp.
In the specific case considered, the most common circuit configuration for push-pull converters uses a transformer provided with three windings, whereof the primary winding has a central tap and a pair of bipolar power transistors of NPN type in common-emitter configuration driven by a resonant system formed by a capacitor and by the primary winding of the transformer itself.
Usually, these converters are supplied with low d.c. voltages and generate high a.c. voltages, used for igniting the fluorescent tubes of cold-cathode fluorescent lamps.
FIG. 1 illustrates a possible circuit embodiment of a converter with current source.
In detail, a push-pull converter 1 comprises a transformer 2 with three windings, namely a primary winding 3, a secondary winding 4, and a tertiary winding 5.
The primary winding 3 has a first input terminal 6 and a second input terminal 7 connected to the drain terminals of a first transistor 8 and of a second transistor 9, respectively, both being of NPN bipolar type and having emitter terminals connected together and to ground. The primary winding 3 moreover has a central tap 10 connected, via an inductor 11, to a supply input 12 (for example, one that supplies a 12-Vdc voltage).
The central tap 10 divides the primary winding into two separate half-windings, namely a first half-winding 3a, connected to the first input terminal 6, and a second half-winding 3b, connected to the second input terminal 7.
The secondary winding 4 has a first output terminal 13, connected, with interposition of a capacitor 14, to a cold-cathode fluorescent lamp 15, and a second output terminal 16 connected to ground.
The tertiary winding 5 has a third output terminal 17 and a fourth output terminal 18, connected to the base terminals of the transistors 8 and 9, respectively.
The push-pull converter 1 further comprises a capacitor 19 connected between the input terminals 6 and 7 of the primary winding 3 and, hence, between the collector terminals of the transistors 8, 9. The base terminals of the transistors 8, 9 are moreover connected to the central tap 10 of the primary winding 3 via respective resistors 20 and 21.
Operation of the push-pull converter 1 is described hereinafter.
Upon turning on the circuit, the d.c. voltage supplied on the supply input 12 generates, in the inductor 11, a current of increasing value, which is supplied, via the resistors 20 and 21, to the bases of the transistors 8 and 9, respectively. In this situation, turning-on of one of the two transistors 8, 9, for example, transistor 8, is obtained. Consequently, there is a current flow in the first half-winding 3a, as well as in the tertiary winding 5, which contributes to maintaining the transistor 9 turned off by drawing current from its base. In this way, the resonant circuit formed by the primary winding 3 and the capacitor 19 is triggered and generates an a.c. voltage between the input terminals of the primary winding 3.
At the end of the turning-on step, the tertiary winding 5 of the transformer 2, which is connected to the bases of the transistors 8 and 9, enables, each time, just one of the two transistors, keeping the other turned off and thus preventing simultaneous conduction of both of them (cross-conduction), which would not enable correct operation of the push-pull converter 1.
In the steady-state condition, the current that traverses the inductor 11 flows, for one half-period, in the transistor 8 and in the first half-winding 3a and, for the other half-period, in the transistor 9 and in the second half-winding 3b. In this way, a variable flow is generated in the core of the transformer 2, which, by concatenating with the turns of the secondary winding 4, induces an electromotive force having a sinusoidal pattern and an adequate amplitude across the cold-cathode fluorescent lamp 15 (for example a sinusoidal voltage with peak amplitude of 800 V is induced).
After the cold-cathode fluorescent lamp 15 has been triggered, the resonance frequency of the circuit is determined not only by the capacitor 19 and by the primary winding 3, but also by the capacitor 14.
The resistors 20 and 21 perform a dual function, in so far as they enable triggering of the circuit in the turning-on step, by enabling the transistor 8 or the transistor 9, and, in addition, supply a fair amount of the base current to the transistor 8 or 9 that is each time turned on during steady-state operation of the push-pull converter 1.
The push-pull converter 1 of the type described has the advantage of enabling driving of fluorescent tubes, which require high a.c. voltages for their ignition, starting from a low d.c. voltage and, moreover, has the advantage of being very robust both in case of open-circuit load and in case of short-circuit on the output, without the addition of any further circuit components.
The above embodiment of the push-pull converter 1 presents however also the disadvantages outlined hereinafter.
The transistors 8 and 9, in common-emitter configuration, must be made as discrete components, in two separate islands. In fact, as illustrated in FIG. 2, the transistors 8, 9 are power transistors of vertical type; in detail, each transistor 8, 9 is formed by a chip comprising an N-type substrate 24, forming the collector, a P-type epitaxial layer 25, forming the base, and an N-type region 26, forming the emitter of the transistors 8, 9. To assemble the transistors 8, 9 in a same package using a same frame 28, it is hence necessary to separate the two collectors, by arranging an insulation region 27 underneath at least one of the two substrates 24. This entails, however, a considerable increase in the production costs.
In addition, a further disadvantage is represented by the fact that, during turning-on and -off of the transistors 8 and 9 current spikes are present both on the base terminals and on the collector terminals and these can give rise to electromagnetic interference (EMI). Such current spikes are due to the storage time of the bipolar transistors, i.e., the time necessary for the extraction of the charges from the base during turning-off of one of the two transistors and simultaneous turning-on of the other; during this time, the charges flow along the tertiary winding of the transformer and thus involve both of the transistors.
Said phenomenon is highlighted in FIG. 3, which shows the plots of the base current Ib, of the collector current Ic, and of the collector-emitter voltage Vce for one of the transistors 8, 9. As may be noted, both the base current Ib and the collector current Ic have current spikes at turning-on and/or at -off of the respective transistor 8, 9.
Finally, the push-pull converter 1 of FIG. 1 envisages use of a transformer provided with three distinct windings, as illustrated in FIG. 4, wherein the windings 3, 4, 5 of the transformer 2 and the corresponding input and output terminals are designated by the same reference numbers as the ones used in FIG. 1, and wherein the transistors 8, 9 are represented as open or closed switches, according to whether they are off or on, respectively. It is evident that the illustrated structure renders the production process complex and expensive.